Data multiplexing for diversity operation

ABSTRACT

In a satellie mobile telecommunications system based on the GSM standard, and using a TDMA frame structure, discontinuous transmission (DTX) mode is used to take advantage of the substantial silences which occur during normal speech. In this mode, traffic channel (TCH) data is not sent, but control of the link between a ground station and a user terminal is maintained by sending control channel data bursts (SACCH) together with silence descriptor (SID) frames. To avoid high peak to mean values of transmission power at the satellite, the emission time of the bursts is controlled so as to be uniform over the available sending opportunities. Conventional techniques for doing this cannot be used in a system which includes diversity operation. Therefore, the burst transmission time at the ground station is set in dependence on a reference provided by the user terminal which is a modified version of the reference generated for contention resolution of random access requests.

The present application is a continuation application of applicationSer. No. 09/959,815, filed Feb. 28, 2002, which application is based onInternational Application No. PCT/GB00/01880, filed May 17, 2000,claiming priority to European Patent Application No. 99303987.4, filedMay 24, 1999, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

This invention relates to data multiplexing in a mobiletelecommunications system, such as a satellite telecommunicationssystem, which includes diversity operation, with particular but notexclusive application to multiplexing of link control data in a systemwhich includes discontinuous transmission over diverse communicationpaths,

BACKGROUND

Terrestrial mobile telecommunications systems are well known and anumber of different systems have developed which operate according todifferent standards, both analog and digital. In Europe and the FarEast, excluding Japan, and elsewhere, the digital Global System Mobile(GSM) network has become popular, whereas in the USA, networks whichoperate according to the IS-41 recommendations such as the AdvancedMobile Phone System (AMPS) and the Digital Advanced Mobile Phone System(DAMPS) are used. In Japan, the Personal Handiphone System (PHS) and thePersonal Digital Communication (PDC) network are in use. More recently,proposals have been made for a Universal Mobile TelecommunicationsSystem (UMTS). These networks are all cellular and landbased but havedifferences in architecture and use different signalling protocols andtransmission frequency bands.

Mobile telecommunication systems have been proposed that use satellitecommunication links between mobile user terminals and conventionalterrestrial networks such as public switched telephone networks (PSTNs)and public land mobile networks (PLMNs). One network known as theIRIDIUM™ satellite cellular system is described in EP-A-0365885 and U.S.Pat. No. 5,394,561 (Motorola), which makes use of a constellation ofso-called low earth orbit (LEO) satellites, that have an orbital radiusof 780 km. Mobile user terminals such as telephone handsets establish alink to an overhead orbiting satellite, from which a call can bedirected to another satellite in the constellation and then typically toa ground station which is connected to conventional land-based networks.

Alternative schemes which make use of so-called medium earth orbit (MEO)satellite constellations have been proposed, with an orbital radius inthe range of 10-20,000 km. Reference is directed to the ICO™ satellitecellular system described for example in GB-A-2 295 296. With thissystem, the satellite communications link does not permit communicationbetween adjacent satellites. Instead, a signal from a mobile userterminal such as a mobile handset is directed firstly to the satelliteand then directed to a ground station or satellite access node (SAN),connected to conventional land-based telephone networks. This has theadvantage that many components of the system are compatible with knowndigital terrestrial cellular technology such as GSM. Also simplersatellite communication techniques can be used than with a LEO network.Reference is directed to “New Satellites for Personal Communications”,Scientific American, April 1998, pp. 60-67, for an overview of LEO/MEOsatellite networks.

Conventional GSM-based systems use a scheme based on a combination oftime and frequency division multiple access to provide communicationchannels. The available bandwidth is divided into a number of carrierfrequencies, each of which is further divided using a TDMA scheme.Speech and data are carried by a number of traffic channels (TCH).

To allow signalling messages to be conveyed along with the user data,each traffic channel is associated with a low rate channel, known as theSlow Associated Control Channel (SACCH). This is used mainly for linkmaintenance and control procedures, such as transmission power control,timing advance control and the transmission of information relating tomeasurement reporting procedures to be performed by the user terminal.

The TDMA frame structure for TCH/SACCH channels according to the GSMSpecifications is shown in FIG. 1. The basic unit of transmission is aseries of about 100 modulated bits which is referred to as a burst.Bursts are sent in time and frequency windows which are referred to asslots. The duration of a slot is referred to as a burst period or BP andin the GSM system lasts 15/26 ms (approximately 0.577 ms). Eight burstperiods are grouped into a TDMA frame, which lasts 120/26 ms(approximately 4.615 ms). Each traffic channel is based on one burstperiod per frame, so that eight TCH channels can be accommodated perframe. In a 26 frame multiframe, which lasts 120 ms, frames 0 to 11 and13 to 24 each carry 8 channels of TCH data, while frame 12 carries SACCHdata, each SACCH burst period providing the necessary signalling for oneTCH channel. Frame 25 is unused. A complete SACCH message or block isdistributed over four multiframes i.e. 480 ms so that 2 SACCH blocks aresent approximately every second.

Each time slot is associated with a unique number referred to as theAbsolute Time Slot Number (ATN). For a 26 frame multiframe with 8 slotsper frame, this runs from, for example, 0 to 207. The GSM specificationsdefine the time slot number (TN) of a particular channel as the ATN mod8, i.e. the remainder when the absolute time slot number is divided bythe number of slots per frame. This is a number in the range 0 to 7specific to a TCH channel. For example, ATN 22 (starting with slotnumber 0) lies in frame 3 and represents a TCH channel with TN=6.

The efficiency of a GSM system is increased by operating indiscontinuous transmission mode (DTX), which reflects the fact that auser only speaks for a proportion of the time during normalconversation. During DTX operation, the traffic channel TCH is not sent.However, signalling is still required to maintain and control the link,so that the SACCH is still transmitted, together with frames known asSilence Descriptor (SID) frames. The purpose of SID frames is totransfer the characteristics of the background noise at the transmitterto the receiver. This feature aims to overcome the disturbance to thelistener which has otherwise been shown to result from the suddendisappearance of background noise when the speaker stops speaking.

During DTX mode, to avoid an uneven load on the base stationtransmitter, with all SACCHs being transmitted at frame 12, the emissiontime for the SACCH bursts can be spread evenly over the empty frames.This can be achieved by multiplexing the SACCH burst according to thetime slot number TN allocated to a particular channel. For example, forthe frame structure described above, the SACCH burst can be transmittedon (ATN div 8=0) for TN=0, (ATN div 8=1) for TN=1, and so on, where adiv b is the quotient when a is divided by b. Therefore, the SACCH burstassociated with TCH channel 0 is emitted in frame 0, and that associatedwith channel 2 in frame 2, as shown in FIG. 2. Therefore, a single SACCHburst and a single SID burst are sent in each of frames 0 to 7 andnothing is emitted in any of the other frames. This scheme does notconstrain the complete SACCH emission to take place during this framenumber. For example, the SACCH to may be multiplexed over, for example,eight frames beginning at a specified frame number.

In a satellite system, the satellite constellation can be configured sothat for any location of user terminal on the earth, more than onesatellite is at an elevation of more than 10 degrees above the horizonand hence two satellites are usually available for communicationconcurrently with the user terminal. The availability of more than onesatellite permits so-called diversity operation in which a trafficchannel can be transmitted between the ground and the user terminalconcurrently via two satellites, in two paths, to mitigate effects ofblockage and fading. Diversity operation is described in GB-A-2 293 725and EP-A-0 837 568.

In systems which support diversity operation, the separate physicalpaths for transmission make up a single logical path, so that theemissions on the diversity paths must correspond to identical encodeddata. For dual path diversity, the user terminal is designed to receivetwo bursts per frame, for example a burst from a first satellite in timeslot TN=0 and a burst from a second satellite at time slot TN=4. Sincethe time slot number TN will be different for the two diversity paths,multiplexing of the SACCH emissions cannot be based on the time slotnumber TN, since the SACCH data would not then arrive at the userterminal within the same frame. For example, using the TN basedmultiplexing scheme explained above, the SACCH burst from the secondsatellite would arrive at the user terminal 4 frames after the burstfrom the first satellite, so introducing significant delays inprocessing the SACCH signals.

In addition, a TN based multiplexing scheme can impair the ability ofthe system to use the time alignment of the SACCH frames as a uniquereference for functions such as measurement reporting and burstalignment in DTX operation. It can also facilitate attacks on theencryption scheme in systems where data in different frames is encryptedusing different encryption keys.

SUMMARY OF THE INVENTION

The present invention aims to overcome the above problems.

According to the present invention, there is provided a method of datamultiplexing in a mobile telecommunications system in which a groundstation is in communication with each of a plurality of user terminalsvia a respective logical channel, each logical channel capable ofcarrying link control data to a respective user terminal over diversecommunication paths using a multiple access scheme which includes timedivision, in which a frame comprises a sequence of data bursts, eachburst being associated with a logical channel, the method comprising:for each channel, setting the data burst transmission time from theground station such that data bursts carrying link control data relatingto the same channel and travelling over diverse paths to the same userterminal, arrive at the user terminal within a predetermined number offrames, and distributing the ground station transmission time of databursts carrying link control data for different channels over aplurality of frames.

In accordance with this method, a user terminal is able to predict thearrival times of link control data for each of the diversity paths. Thedata burst transmission time can be set so that bursts carrying linkcontrol data relating to the same channel and travelling over diversepaths to the same user terminal, arrive at the user terminal within thesame frame, so that there is no delay in processing the two identicalparts of the link control data. In discontinuous transmission mode,arrival of the link control data from diverse paths in the same frameminimises the number of frames in which data needs to be sent.

Advantageously, the ground station burst transmission time can be set inaccordance with transmission control information provided by the userterminal. To avoid the need for additional signalling channels, thisinformation can be part of the random reference used by the groundstation for contention resolution of random access requests from aplurality of user terminals. Alternatively, the transmission controlinformation can be generated at the ground station.

Multiplexing the data bursts in accordance with the method of theinvention provides a staggering of the data burst transmission times atthe ground station, so limiting the peak transmission power required atthe ground station. This in turn avoids high peak to mean values oftransmission power, i.e. power ripple, at the satellite which, in asatellite system, significantly limits the number of traffic channelscapable of being supported.

According to the invention, there is further provided a link controlsignal for maintaining link control between a user terminal and a groundstation in a mobile telecommunications system in which a ground stationis in communication with each of a plurality of user terminals via arespective logical channel, each logical channel capable of carryinglink control data to a respective user terminal over diversecommunication paths using a multiple access scheme which includes timedivision, in which a frame comprises a sequence of data bursts, eachburst being associated with a logical channel, the signal beingconfigured such that data bursts carrying link control data relating tothe same channel arrive at the user terminal within a predeterminednumber of frames, and such that data bursts carrying link control datafor different channels are distributed over a plurality of frames.

According to the present invention, there is also provided a userterminal for use in a mobile telecommunications system in which a groundstation is in communication with each of a plurality of user terminalsvia a respective logical channel, each logical channel capable ofcarrying link control data to a respective user terminal over diversecommunication paths using a multiple access scheme which includes timedivision, in which a frame comprises a sequence of data bursts, eachburst being associated with a logical channel, the user terminalcomprising means for providing transmission control information to theground station to control the setting of data burst transmission timesat the ground station, in dependence on which data burst transmissiontimes are set at the ground station such that data bursts carrying linkcontrol data relating to the same channel and travelling over diversepaths to the same user terminal, arrive at the user terminal within apredetermined number of frames, and such that the ground stationtransmission time of data bursts carrying link control data fordifferent channels is distributed over a plurality of frames.

In addition, the present invention also provides a ground station in amobile telecommunications system in which the ground station is incommunication with each of a plurality of user terminals via arespective logical channel, each logical channel capable of carryinglink control data to a respective user terminal over diversecommunication paths using a multiple access scheme which includes timedivision, in which a frame comprises a sequence of data bursts, eachburst being associated with a logical channel, the ground stationcomprising means for setting data burst transmission times in dependenceon transmission control information, such that data bursts carrying linkcontrol data relating to the same channel and travelling over diversepaths to the same user terminal, arrive at the user terminal within apredetermined number of frames, and such that the ground stationtransmission time of data bursts carrying link control data fordifferent channels is distributed over a plurality of frames.

The transmission control information can be provided by the userterminal or generated at the ground station.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 illustrates a prior art TDMA frame structure for TCH/SACCHchannels according to the GSM specification.

FIG. 2 illustrates a prior art SACCH burst for channel 2 in frame 2.

FIG. 3 is a schematic diagram of a satellite telecommunications systemtogether with a local, land-based mobile telecommunications system, inaccordance with the invention;

FIG. 4 is a schematic diagram of the spot beam pattern produced by oneof the satellites on the earth;

FIG. 5 illustrates schematically different beam types and theirassociated cellular areas and Z-arcs, from the pattern shown in FIG. 4;

FIG. 6 is a schematic frequency/time diagram illustrating time slots forthe frequency diverse TDMA transmission scheme;

FIG. 7 is a schematic diagram showing the TCH/SACCH burst formatting;

FIG. 8 is a schematic block diagram of the circuits of satellite 3 a;

FIG. 9 is a schematic diagram of the cell pattern produced by the spotbeams of satellites 3 a, 3 b;

FIG. 10 is a schematic diagram of a mobile user terminal;

FIG. 11 is a schematic block diagram of the circuits of the userterminal shown in FIG. 10;

FIG. 12 is a schematic block diagram of SBS 1 shown in FIG. 3;

FIG. 13 is a schematic diagram of TDMA signals transmitted between thesatellites and the user terminal;

FIG. 14 is a schematic diagram showing the relationship of atransmission path between a user terminal and satellite, referenced to aZ-arc;

FIG. 15 is a flow diagram illustrating the operation of the SACCHmultiplexing 20 scheme; and

FIG. 16 is a schematic diagram showing the structure of the DTX signalin accordance with the invention.

DETAILED DESCRIPTION

Overview of Network

Referring to FIG. 3, a schematic block diagram of a satellite mobiletelecommunications network is shown corresponding to the ICO™ network. Amobile user terminal UT 1 in the form of a mobile telephone handset cancommunicate on a radio channel over a communication path 1 a, 2 a via anearth orbiting satellite 3 a with a land-based satellite access node SAN1. As shown schematically in FIG. 3, SAN 1 is provided with a dishantenna arrangement that includes antenna 4 a which can track theorbiting satellite 3 a.

A number of the satellite access nodes SAN 1, 2, 3, etc are connectedtogether to form a backbone network 5, which is connected through anumber of gateways GW 1, 2, 3, etc to conventional land-based telephonenetworks. For example, the gateway GW 1, is connected to a land-basedpublic switched telephone network (PSTN) 6, which permits connection tobe made to a conventional telephone set 7. The gateway GW 1 isadditionally connected to a public switched data network (PSDN) 8 and apublic land mobile network (PPAN) 9. Each of the gateways GW 1, 2, 3 maycomprise existing International Switching Centres (ISCs) or mobile toswitching centres (MSCs) of the type used in GSM mobile networks.

As shown in FIG. 3, the handset UT 1 is a dual mode device which canalso communicate with the conventional land-based mobile network PLMN 9,that is shown schematically to include a transceiver station 10 whichestablishes a duplex link 11 with the user terminal UT 1. In thisexample, the PLMN 9 is a GSM network. Thus the user can for example roamto the satellite network when out of range of the PLMN 9.

For a fuller understanding of GSM, reference is directed to the variousGSM Recommendations issued by the European Telecommunications Institute(ETSI). Also reference is directed to “The GSM System for MobileCommunications” supra, for a more readable overview.

The satellite network is designed to provide world-wide coverage and thesatellite 3 a forms part of a constellation of satellites, which may bearranged in several orbits. The satellites may be arranged in a MEOconstellation, for example with an orbital radius of 10,390 km, althoughthe invention is not restricted to a particular orbital radius. In oneexample, two orbits of five satellites are used, which can be shown toprovide coverage of a major part of the surface of the earth, in whichfor a 100 satellite elevation angle, one satellite can be accessed bythe mobile handset all of the time and two satellites can be accessedfor at least 80% of the time, thereby providing more than one concurrentcommunication path to the user terminal from a particular SAN.Additional satellites may be included in the constellation in order toprovide redundancy.

In this embodiment, two satellites 3 a, 3 b of the constellation areshown in a common orbit and the satellites are tracked by the antennaarrangement 4 of each SAN. The antenna arrangement 4 for each SAN mayfor example include five dish antennas to track satellites individually,although only two of the dish antennas 4 a, 4 b for SAN 1 are shown inFIG. 3 and only one dish is shown for the other SANs, in order tosimplify the drawing. The dish antennas permit diverse communication topaths to be established between the SANs and an individual user terminalvia different satellites. In this example, the first communication path1 a, 2 a is set up between dish antenna 4 a of SAN 1 and user terminalUT 1 via satellite 3 a, and a second communication path 1 b, 2 b is setup between dish antenna 4 b of SAN 1 and user terminal UT 1 viasatellite 3 b, thus providing first and second diverse paths forconcurrent communication of signal traffic between the satellite and theuser terminal.

The SANs are spaced around the earth in order to provide continuouscoverage. In the example shown, SAN 1 may be located in Europe whereasSAN 2 may be located in Africa, SAN 3 in America and other SANs may belocated elsewhere.

SAN 1 consists of a satellite base station SBS 1 which is coupled to thedish antenna arrangement 4 for tracking the satellites, the SBS 1including transmitter and receiver circuits with amplifiers,multiplexers, de-multiplexers and codecs, which will be described inmore detail later. A mobile satellite switching centre MSSC 1 is coupledto SBS 1 and includes a satellite visitor location register VLR_(SAT) 1.MSSC 1 couples communication signals to the backbone network 5 and tothe SBS 1, so as to allow individual telephone calls to be establishedthrough the backbone network 5 and the duplex communication link 1 a, 2a via the satellite 3 a, to the mobile terminal UT 1. Also, MSSC 1 isconnected to the gateway GW I so as to provide a connection to PLMN 9,PSDN 8 and PSTN 6. It will be understood that all the SANS are ofsimilar construction with a respective VLRSAT to maintain a record ofthe subscribers registered.

In FIG. 3, the SAN 2 is shown communicating with user terminal UT 2 viasatellite 3 b. For further details of the network, reference is directedto GB-A-2 295 296 and EP-A-0869628.

The satellites 3 a, 3 b are in non geo-stationary orbits and comprisegenerally conventional hardware such as the Hughes HS 601. They mayinclude features disclosed in GB-A-2 288 913.

Each satellite 3 a, 3 b is arranged to generate an array of radio beamseach with a footprint on the earth beneath the satellite, each beamincluding a number of different frequency channels and time slots asdescribed in GB-A-2 293 725. The beams thus provide adjacent cellularareas which correspond to the cells of a conventional land-based mobiletelephone network. Referring to FIG. 4, in the ICO™ satellite mobiletelephone system, each satellite 3 a, 3 b produces a fixed pattern of163 spot beams, with the shapes of the spot beams varying as a result ofthe curvature of the earth to produce 19 different beam types (B0-B18),as shown in FIG. 5.

The satellites are controlled by means of a satellite control centre(SCC) 12 and a telemetry tracking and control station (TT&C) 13, whichare connected to a network management centre (NMC) 14 through a digitalnetwork 15 that is coupled to the backbone network 5. The SCC 12 and theTT&C 13 control operations of the satellites 3 a, 3 b, e.g. for settingthe general transmission power levels and transponder input tuning, asdirected by the NMC 14. Telemetry signals for the satellites 3 a, 3 bare received by the TT&C 13 and processed by the SCC 12 to ensure thatthe satellites are functioning correctly.

Channel Configuration

During a telephone call, each of the user terminals communicates with arespective SAN via the satellites. A full duplex communication path isprovided between the UT and the SAN. As referred to herein,communication from the SAN to the UT via the satellite is referred to asa “downlink”, and communication directed from the UT via the satelliteto the SAN is referred to as an “uplink”. As will be explained in moredetail hereinafter, the signals may travel over diverse paths between UT1 and SAN 1 via satellite 3 a or 3 b or both of them concurrently.

The general configuration of the transmitted signals is similar in somerespects to those used for conventional GSM transmissions in a PLMN andmakes use of a frequency diverse time division multiple access (TDMA)scheme. For speech transmission, data is sent on a traffic channel TCH.Each TCH is provided with an associated slow-rate control channel orSACCH. The configuration of these channels in the TDMA scheme will nowbe described in more detail. The basic unit of transmission between theSAN and UT is a series of about 120 modulated symbols, which is referredto as a burst. Bursts each have a finite duration and occupy a finitepart of the radio spectrum. Thus, they are sent in time and frequencywindows which are referred to as slots. The slots are positioned e.g.every 25 kHz and recur in time every 40/6 ms (6.667 ms). The duration ofeach slot is referred to as a burst period or BP. A graphicalrepresentation of a slot in the time and frequency domain is shown inFIG. 6.

Referring to FIG. 7, each TCH consists of one slot every frame (6 BP)and comprises a cycle of 25 slots over a 25 frame multiframe. The SACCHis multiplexed over 12 consecutive slots, with, for example, 16 bits ofSACCH data per slot. Two SACCH blocks are transmitted for every 25 slotcycle, i.e. two blocks per second. One slot in the 25 slot cycletherefore contains no SACCH data, but only TCH data with an extendednumber of sync bits.

It will be appreciated that with this configuration, 6 TCHs can beinterleaved due to the fact that each TCH consists of a slot every 6 BP.The resulting interleaved structure thus provides a 40 ms frame of 6TCHs every 6 BP.

In addition to the channels TCH/SACCH for the individual UTs, a numberof common control channels are defined generally based on a 25 time slotTDMA structure. A downlink broadcast control channel BCCH is broadcastfrom each satellite to all UTs within a particular cell. The BCCHprovides information which identifies the cell to the UT, which isreceived by the UT in idle mode i.e. when no call is being made. As eachcell has its own BCCH, the relative signal strengths of the BCCHs at theUT can be used to determine which cell is to be used for TCH/SACCHcommunication with the UT. Other system information may be transmittedto the UTs of a particular cell in the BCCH in a similar manner to GSM.The BCCH message consists of, for example, one burst every 25 BP.

A common downlink paging logical channel PCH is provided for each cell.This is used to transmit paging messages to a UT, for example a callannouncement message to alert the UT to an incoming call. Also an accessgrant logical channel AGCH indicates to the UT a channel allocated bythe network, to be used by the UT for speech or data communication(TCH/SACCH). The PCH may consist of 1 to 10 slots every 25 BP and theAGCH may consist of e.g. 2, 4, 6, 8, 10 or 12 slots every 25 BP.

In addition to these downlink common channels, there is an uplink commonchannel which allows UTs within a cell to transmit channel accessrequests to the network, when it is desired to make a call from the UTor in response to a call announcement message on the PCH channel. Theserequests thus occur essentially randomly in time and the channel isaccordingly called the random access channel RACH. The RACH consists ofe.g. 2 slots every 3, 4 or 5 BP.

The SAN 1 to satellite 3 a, 3 b uplinks are located in a frequency bandin the region of 5 GHz and the corresponding downlinks are in the regionof 7 GHz. The satellite 3 a, 3 b to UT 1 downlinks are in the region of2.1 GHz and the corresponding uplinks are in the region of 1.9 GHz,although the invention is not restricted to these frequencies. In thisexample, the individual TCH/SACCHs and BCCHs are assigned constantindividual 25 KHz wide frequency bands to provide a TDIMA slot sequenceas explained with reference to FIGS. 6 and 7.

Satellite

A schematic diagram of the major signal processing components of eachsatellite is given in FIG. 8. Signals transmitted from one of the SANSare received by antenna 16 and directed to adetector/multiplexer/de-multiplexer circuit 17. It will be understoodthat the signal transmitted from the SAN to the satellite contains alarge number of TCH/SACCHs that are to be directed to individual UTs bythe satellite. To this end, the satellite includes an array 18 of aplurality (for example 163) of antennas that produce individual spotbeams that correspond to a cellular configuration as previouslydescribed. A beam forming processor circuitry configuration 19 _(down)receives the various TCH/SACCHs that are de-multiplexed by circuit 17and assembles them into multiplexed signals directed on 163 outputs tothe spot beam antennas 18.

For signals on the uplink from the individual UTs to the SAN, thevarious transmissions are received by the spot beam antennas 18 anddirected to processing circuitry 19 _(up) which combines the variouschannels and passes them to the multiplexer configuration in circuit 17for transmission through the antenna 16 to the SAN. It will beunderstood that the foregoing description of the satellite circuitry isschematic and for further details, reference is directed to GB-A-2 288913 supra.

An example of cells C0-C6 produced by the footprints of seven of thespot beams from antenna 18 of satellite 3 a is shown in FIG. 9. Also,one of the spot beams from satellite 3 b is shown, which produces cellC7. The other beam footprints are omitted for purposes of clarity. Thediverse communication paths between SAN 1 and UT 1 are shown. Aspreviously explained, one path 1 a, 2 a extends between antenna 4 a andUT 1 via satellite 3 a. The other path 1 b, 2 b extends between antenna4 b and UT 1 via satellite 3 b. These diverse paths make use of cells C0and C7 of the satellites 3 a and 3 b.

User Terminal (UT 1)

The mobile user terminal UT 1 is shown in more detail in FIGS. 10 and11. It comprises a hand held device which is generally similar to amobile telephone used for conventional terrestrial GSM networks. It ispowered by a rechargeable battery and is configured to operate eitherwith the local terrestrial cellular network or to roam to the satellitenetwork. Thus, in the example shown in FIG. 3, the mobile handset UT 1can operate either according to a land-based GSM protocol or accordingto the satellite network protocol. As shown in FIG. 10, the handsetcomprises a microphone 20, a speaker 21, a battery 22, a keypad 23,antennas 24, 24′ and a display 25. The handheld unit UT 1 also includesa subscriber identification module (SIM) smartcard 26. The circuitconfiguration of the handset UT 1 is shown in block diagrammatic form inFIG. 11. The SIM card 26 is received in an SIM card reader 27 coupled toa controller 28, typically a microprocessor. The microphone and speaker20, 21 are coupled to first and second codecs 29, 29′ coupled toconventional radio interfaces 30, 30′ connected to the antennas 24, 24′so as to transmit and receive communication signals, in a manner wellknown per se. The handset is capable of dual mode operation, with thecodec 29, radio interface 30 and antenna 24 being used with thesatellite network, and corresponding circuits 29′, 30′ and 24′ beingused with the GSM PLMN 9. Amongst other things, the controller 28 canmonitor the quality of received signals from the satellite network, aswill be explained in more detail later.

Satellite Base Station (SBS 1)

The configuration of the satellite base station SBS1 at SAN1 will now bedescribed in more detail with reference to FIG. 12. SBS 1 comprises anetwork interface 31 providing an interface to MSSC 1, a modulator/poweramplifier system 32, first and second feed units 33 a, 33 b for the dishantennas 4 a, 4 b, a demultiplexer 34, a signal processing unit 35 and acontroller 36. As previously explained, SAN 1 may include five dishantennas and the connections for only two of them are shown in FIG. 12in order to simplify the explanation.

The interface 31 receives communication signals from MSSC 1, routed fromone of the various networks either through gateway GW 1 (FIG. 3) or viathe backbone network 5. The interface 31 reformats the signals fortransmission to the UTs. Several calls are simultaneously supplied tothe modulator/power amplifier system 32 and an interleaved TCH/SACCH isproduced under the control of controller 36. A number of TCH/SACCHs areproduced concurrently on different frequency bands and are fed to eitherone or both of the antennas 4 a, 4 b for onward downlink transmissionvia the satellites to individual user terminals. The feed units 33 a, 33b feed the signals to the antennas 4 a, 4 b.

The antennas also receive uplink signals from the satellites and eachfeed unit 33 a, 33 b includes power combiners for combining the signalsfrom the modulator/power amplification system 32 and a circulator forisolating the demultiplexer 34 from the combined signals and directingthe combined signals to the respective antennas 4 a, 4 b.

Uplink signals received from the satellites are directed by the feedunits 33 a, 33 b to the demultiplexer 34, which demultiplexes receiveduplink TCH/SACCHs and feeds the resulting signals via the signalprocessing unit 35 to the interface 31 for onward transmission to MSSC1. The signal processing unit 35 extracts data from the receivedsignals, which are fed to the controller 36 which in turn providescontrol data for UT 1, which is modulated by modulator 32 onto signalstransmitted to the UT, as will be described in more detail later.

Service Provision

The described network can provide service to subscribers in a number ofdifferent 20 ways. For example, communication may be provided from UT 1to UT 2 using the satellite backbone network 5. Alternatively, telephonecommunication can be established between the telephone set 7 and UT 1either via SAN 1 and the satellite network, or, through PLMN 9, antenna10 and link 11. For further description of service options, reference isdirected to EP-A-0869628.

In the following, a telephone call established between telephone set 7and UT 1 through SAN 1 and satellites 3 a, 3 b will be considered inmore detail.

Radio Link Configuration

Referring to FIG. 13, the TDMA frames which make up the TCH/SACCH, needto achieve synchronisation in order to achieve a satisfactory radio linkbetween the SAN and UT 1. In the network described herein, link controlis carried out referenced to each satellite, to ensure that integrity ofthe TDMA frame structure is maintained at the satellite, notwithstandingtransmission delays to and from the satellite and Doppler shifts infrequency due to orbital motion of the satellite. It can be shown thatchoosing the satellite as the reference position reduces the overallcomplexity of the link control arrangements.

The issues associated with timing delays and Doppler shift will beconsidered in turn. Considering timing delays, for a terrestrial,cellular, mobile telephone network, the signal typically takes a fewmicroseconds to travel from a base transmitter site antenna to a mobilestation. Consequently, the transmitted TDMA time slot pattern isgenerally retained at the mobile station. A typical round trip delay fora 35 km cell is of the order of ¼ slot. However, in a satellite mobiletelephone system with the satellites 3 a, 3 b in a medium earth orbit,the time taken for signals to travel between a satellite 3 a, 3 b and aUT 1 can be longer than a time slot. In the case of a satellite orbitingat 10390 km, the one way propagation time to the satellite's nadir(where the satellite is seen at an elevation of 90° on the Earth) isapproximately 34.5 ms, which is comparable with the previously described40 ms frame duration. This situation is further complicated by therebeing a significant difference between the path length to the cell atthe nadir and the path lengths to the cells at the edge of thesatellite's footprint. For a satellite orbiting at 10390 km, the one waypropagation time to a UT which sees the satellite at 0° elevation isapproximately 51.6 ms. This leads to significant differences in thetimes at which bursts arrive at UTs and in the times when UTs arerequired to transmit in order for their signals to fit in with the TDMAframe structure at the satellite.

As previously mentioned, the up and downlink traffic channel signals areto be nominally synchronised in timing and carrier frequency at thesatellite. In relation to timing synchronisation, this means that pathdelay variations on links to individual UTs need to be compensated aswill now be explained.

Referring to FIG. 13, the transmission of successive downlink TDMAbursts 40, 41 from satellites 3 a,b over paths 1 a,b, to UT 1 is showntogether with uplink bursts 42,43 from the UT to the satellites. Thebursts in this example have a duration of 40/6 ms (6.667 ms) aspreviously discussed, and form part of the TCH/SACCH. At UT 1, each 40ms frame N is defined as two 20 ms diversity windows w1, w2 forcommunication with the two satellites 3 a,b. Considering the downlinkburst 40 transmitted from satellite 3 a, it is transmitted at a timedetermined by the synchronisation pattern that is maintained atsatellite 3 a, over path 1 a to UT 1 and received within the 20 msreception window w1. A transmission delay T_(p) occurs over the path 1 abetween the satellite 3 a and UT 1. The user terminal UT 1 is configuredto transmit an uplink burst 42 after a nominal, predetermined time delayD following reception of the downlink burst 40. The uplink burst 42 alsois subject to the transmission delay T_(p) as it travels to thesatellite 3 a. The time delay D needs to be selected so that the uplinkburst 42 is received at the satellite 3 a at a time which fits into theperiodic pattern of the TDMA structure at the satellite 3 a, in order tomaintain synchronisation at the satellite. In this example, the timedelay D is nominally 30 ms but is subject to an offset ε_(t) which maybe a positive or a negative quantity, to account for variations inlocation of the UT, as will be explained hereinafter.

Communication between the satellite 3 b and the user terminal UT 1occurs in a similar way. The downlink TDMA burst 41 is received indiversity window w2 at UT 1 from satellite 3 b over path 1 b and theuplink burst 43 is transmitted to the satellite 3 b by the UT after adelay D+ε_(t), so as to maintain synchronisation at the satellite 3 b.Suitable values for ε_(t), for each satellite path are selectedindividually, as explained hereinafter.

Compensation to achieve the necessary synchronisation is carried out byreferencing the timing for bursts transmitted on the path between thesatellite and the UT to one of a number of different individualtransmission delay values T₀ for which a zero offset ε_(t), is required.These different, individual transmission delay values map onto thesurface of the Earth as arc-shaped contours Z as shown in FIG. 5. Thelocation of the UT is considered in relation to one of these contours.When the UT is located on the contour, the timing delay corresponds tothe individual value associated with the contour and the value of thetiming offset ε_(t), needed is zero in order to maintain synchronisationof the uplink and downlink bursts at the satellite. When the UT ispositioned away from the contour, a timing offset ε_(t), is neededrelative to the value at the contour, to compensate for the shorter orlonger transmission path length between the satellite and the UT. Sincethe arc-shaped contours Z of constant time delay require, by definition,zero timing offset, the zero-offset arcs are referred to as Z-arcs.

In this example, each Z-arc delay value defines a path delay class whichlies within +/−1.4 ms of that value, so that 17 delay classes cover the163 spot beams. FIG. 5 shows Z-arcs Z_(n) superimposed onto the spotbeam types. Beam types 0 to 5, for example, have a path with delayvariation of only 2.2 ms, and are therefore covered by a single Z-arc.In beam spots with more than one delay class, dotted arcs show theboundaries between delay classes.

The relationship between the timing offset ε_(t), the path delay T_(p)to the UT and the delay T₀ associated with an individual Z-arc can beseen from FIG. 14 which shows the transmission delays over the pathsassociated with the satellite 3 a. It can be seen that the timingoffset, when referenced to the Z-arc, is given by:ε_(t)=2(T _(p) −T ₀)  (1)

The factor of two deals with the fact that compensation is needed forboth the uplink and the downlink path between the UT and the satellite.

The offset ε_(o) which, in this example has a maximum value of +/−2.8ms, can be calculated by SBS 1 and periodically sent to UT 1. The SBS 1knows the Z-arc value corresponding to the delay class to which the UT 1has been allocated. It can determine the delay between itself andsatellite 3 a by sending and receiving loop-back transmissions. Otherdelays within the system can be measured or details obtained from therelevant equipment manufacturer, for example, in the case of satellitetransponder delays, from the satellite manufacturer, and thisinformation supplied to the SBS 1. Since the SBS 1 knows the variousoffsets (i.e. the delay D and others) applied by the UT between thereceived and transmitted bursts, it can use a received burst from the UTto calculate the actual path delay between the SBS and UT 1.

The way in which a UT is allocated a traffic channel will now bedescribed. UT 1 first acquires system time and frequency from a BCCH. Toinitiate radio access, UT 1 sends a channel request message bytransmitting a formatted burst in a prescribed time slot on the RACHcarrier frequency. As part of the contention resolution proceduresrequired as a result of the use of a random access channel, the UT mustbe able to correlate a channel assignment from the network with its ownrequest. For this purpose, part of the information content of thechannel request message, for example five bits, is chosen randomly bythe UT, which significantly reduces the probability that two Uts willsend identical channel access request messages during the same slot.

SBS 1 searches for and acquires the UT RACH burst timing. As explainedabove, by knowing the received RACH time and various fixed offsets, theSBS can work out the actual path delay between the satellite 3 a and theUT and therefore decide which Z-arc the UT should use, if the beam hasmore than one. The assignment of a traffic channel TCH/SACCH and initialpre-corrections are sent to the UT on the AGCH, together with the randomreference initially sent by the UT and the absolute time slot number(ATN) at which the channel request message was sent. The randomreference and ATN enable each UT receiving the AGCH to check that thechannel assignment corresponds to its request.

During the traffic exchange on the TCH/SACCH, SBS 1 pre-corrects thedownlink frame timing of the bursts transmitted from the SAN 1 to thesatellite so that the bursts, on arriving at the satellite, achieve thedesired synchronisation pattern, that results in zero frame timingoffset on Earth along the Z-arc. The UT locks to the correspondingreceived burst timing from the satellite 3 a and pre-corrects andtransmits its uplink burst 30 ms +/− up to 2.8 ms later (D+ε_(t)) so asto maintain nominally zero timing error at the satellite 3 a. The SBS 1periodically, for example, once per minute, sends new timing offsets toUT 1, based on continuing measurement of uplink burst timing at SBS 1.Thus referring to FIG. 12, the signal processor 35 measures timingerrors in the received uplink bursts and based on this information, thecontroller 36 periodically encodes updated values of ε_(t) inSACCH_(down) using modulator 32 for transmission to UT 1.

A similar process occurs for the link established through satellite 3 b,between SAN 1 and UT 1, for diversity operation. The bursts of theTCH/SACCH sent through satellite 3 b are subject to a correspondingpre-compensation process in which SBS 1 computes timing offsets relativeto a Z-arc appropriate for satellite 3 b. The timing offsets aretransmitted to UT 1 and used by it to pre-compensate uplink bursts of tothe TCH/SACCH routed through satellite 3 b. Downlink bursts from the SANto the satellite 3 b are pre-corrected relative to the Z-arc forsatellite 3 b and also to maintain synchronism of the diversity windowsw1, w2 at UT 1 for the satellites 3 a, 3 b. The periodic updatingprocess will be described in more detail hereinafter.

As previously mentioned, a Doppler shift occurs in the frequency of thebursts transmitted between each satellite 3 a,b and UT 1, which unlesscorrected, can result in bursts shifting in the frequency domain fromone allocated slot to another. Compensation in the frequency domain iscarried out by defining Z-arcs of zero Doppler frequency offset andperforming compensation at UT 1 and at SBS 1 in a similar manner asdescribed herein for timing errors, but in respect of errors caused byDoppler shifts.

Diversity Operation

As previously explained, a call established between SAN 1 and UT 1 canbe routed via satellite 3 a and satellite 3 b over diverse paths 1 a, 2a and 1 b, 2 b. The call can be routed over both paths concurrently toachieve diversity. This has the advantage that if one of thecommunications paths is subject to blockage or fading the other path canstill provide good communication, thereby assuring a good signalquality.

The system described herein can operate in a number of differentdiversity modes including full diversity and partial diversity, whichwill be considered in more detail hereinafter. The link control processis configured to allow frequency .and timing compensation to beperformed whichever diversity mode is selected.

Full Diversity Mode

When in full diversity mode, a TCH/SACCH transmitted on the downlinkfrom SAN 1 to UT 1 is concurrently transmitted over both of the paths 1a, 2 a; 1 b, 2 b, both for the uplink and the downlink. Considering thedownlink, a TCFUSACCH_(down) is formed in the SBS 1 by themodulator/power amplifier system 32. Alternate TDMA frames which make upthe TCH/SACCH are directed to the antennas 4 a, 4 b respectively viafeed units 33 a, 33 b. The antennas feed the signal bursts on differentfrequencies to the satellites 3 a, 3 b respectively. For example, the toTCH/SACCH for UT 1 uses one of the two TDMA slot pairs 0 and 3, 1 and 4or 2 and 5 for the two diversity links.

Considering now operations at UT 1, TCH/SACCH_(down) is received via thetwo satellites 3 a, 3 b in two frequency bands and the circuit shown inFIG. 11 receives these signals on antenna 24 and feeds them to the radiointerface 30. The codec 29 assembles the bursts received in the twodifferent frequency bands into a signal train which is then convertedinto an analog signal and fed to the speaker 21.

The controller 28 measures the bit error rate (BER) individually for thereceived TDMA bursts for each of the paths 1 a, 1 b from the satellite 3a, 3 b. Also, the controller 28 measures the Doppler shift δf_(a),δf_(b) in the signals received from the satellites 3 a, 3 b andcalculates the difference between them Δf=(δf_(a)−δf_(b)). Furthermore,the controller 28 detects any timing errors δt_(a), δt_(b) for the TDMAbursts received on each of the two paths 1 a, 1 b from the satellitesand calculates the 25 difference between them Δt=(δt_(a)−δt_(b)). Thislink control data LD (Δf & Δt) is encoded, using codec 29 of UT 1 (FIG.11) in the channel SACCH_(up), associated with the uplink TCH_(up).

In full diversity mode, the TCH/SACCH_(up) is transmitted from UT 1 toSAN 1 via both satellites 3 a, 3 b and so the link control data in theSACCH_(up) is received by the two dish antennas 4 a, 4 b and is directedto the demultiplexer 34 and signal processing unit 35.

As previously explained, the signal processor 35, monitors timing errorsin the uplink bursts received via path 1 a, 2 a and based on thisinformation, the controller 36 periodically instructs modulator 32 tosend an updated value of the timing offset ε₁ for path 1 a (satellite 3a) to UT 1 via the downlink SACCH_(down). A corresponding updating ofthe Doppler offset ε_(d) for path 1 a is carried out in a similarmanner.

This updating process also needs to be carried out in respect of thetiming offsets for path 1 b (satellite 3 b). The link control data LDtransmitted in SACCH_(up) is received by the processor 35 and used tocompute updated values of offsets for path 1 b, referred to herein asε₁(b) and ε_(d)(b), based on the current value of the offsets ε₁, ε_(d)for path 1 a, referred to herein as ε₁(a) and ε_(d)(a), and differentialdata Δt=(Δt_(a)−εt_(b)) and Δf=(εf_(a)−εf_(b)).

Controller 36 periodically instructs modulator 32 to encode the updatedvalues of ε₁ (b), ε_(d) (b), ε₁ (a) and ε_(d) (a) in SACCH_(down) linkcontrol data for the UT, to be transmitted to UT 1. The advantage ofthis updating method is that it is possible to continue to perform theupdating process with respect to path 1 b when propagation conditionsare such that either path 1 a or 1 b degrade the reception of burstsfrom satellite 3 b such that the burst cannot be used for derivingtiming offset control.

It would also be possible to monitor the two uplink paths via the twosatellites 1 a, 1 b separately.

Partial Diversity Mode

In this mode, the downlink channel TCH/SACCH_(down) is transmitted to UT1 from SAN 1 via both satellites 3 a, 3 b, as in the full diversity modepreviously described. However for the uplink, TCH/SACCH_(up) istransmitted via only one of the satellites 3 a, 3 b. This has theadvantage of saving battery power at UT 1. The best uplink path isselected by consideration of the BER for the downlink channelsindividually, detected by controller 28 of UT 1 shown in FIG. 11.

Thus in the partial diversity mode, only one uplink path is availableand so the uplink paths cannot be independently monitored at the SBS toupdate the offsets ε₁(a), ε_(d)(a) and ε_(t)(b), ε_(d) (b).

UT 1 is therefore configured to transmit link control data LD relatingto both of the paths 1 a, 1 b, in one of the uplink paths to the SBS 1.Thus, for example if the path 1 a through satellite 3 a is selected forthe partial diversity mode, the processor 35 at SBS 1 can monitor errorsin the uplink and calculate the offsets ε₁ (a), ε_(d) (a) accordingly.The values of ε_(t) (b) and ε_(d) (b) can then be calculated by theprocessor 35 as described above. Conversely, if satellite 3 b isselected for the uplink, SBS 1 can monitor errors in this uplink andcalculate the offsets ε_(t) (b), ε_(d) (b), from which ε_(t) (a) andε_(d) (a) can be calculated using differential link data LDΔt=(δt_(a)−δt_(b)) and Δf=(δf_(a)−δf_(b)).

This arrangement thus has the advantage that link control data pertinentto both of the paths is determined by UT 1 and fed back on only one ofthe uplink paths via one satellite only. If the selected path for theuplink changes, no change is required to the configuration of the linkcontrol data which is configured to provide information about both ofthe paths and thus can be transmitted on either uplink path.

The described examples of diversity operation use two transmissionpaths. However, more paths can be used, i.e. three or more. Thus, thediversity modes can be generalised to include m paths with an optionalselection of n paths where m>n. It will be understood that the inventioncan also be applied to this generalised configuration.

Discontinuous Transmission (DTX)

As mentioned above, the performance of a mobile communications systemcan be improved by operating in DTX mode, to take advantage of the factthat speech is substantially discontinuous.

In this mode, the TCH is not sent during voice pauses, but theSACCH_(down) signal still needs to be sent to maintain link control, forexample, to permit timing and frequency compensation to be performed asdescribed above. In addition, as previously mentioned, silencedescriptor (SID) frames are sent during voice pauses. The SACCH data canbe transmitted fully in two pre-planned frame positions in eachmultiframe. However, when considered over all channels, this solutiongives rise to significant peak power requirements and undesirable powerripple at the satellite. In accordance with the invention, alternativeapproaches which avoid this problem are described in detail below.

The way in which a UT is allocated a traffic channel when the DTX modeis to be to used will now be described with reference to FIG. 15. Asexplained above, to initiate radio access, the UT 1 sends a randomreference (REF) to SBS 1 as part of the channel request message. Thisreference can be used to determined initial multiplexing of the SACCHfor all diversity paths. The SACCH frame assignments can be updated athandover or channel reassignment, or can be left as allocated at theinitial assignment.

At the UT 1, at step s1, a random reference REF is first generatedcomprising y bits, for example 7 bits. At step s2, the UT controller 28constrains a sufficient number of bits of the y-bit random reference REFto be in the range 0 to n−1, where n is the total number of frames overwhich the SACCH is to be distributed, to produce a constrained referenceCREF. For example, where 12 frames are available for each SACCH block,n=12 and the 192 bits (16×12) of SACCH can be transmitted to arrive in asingle frame at any one of frames 0 to 11 dependent on the constrainedreference CREF.

For example, the least significant [log₂ (n) bits] of REF, where [x]represents a rounding up of x, are constrained to provide the referenceCREF. Assuming that n=12, then log₂(n)≈3.6 and [log₂(12)]=4. Therefore,the least significant 4 bits of the y-bit reference REF are constrainedat the UT controller 28 to represent the frame numbers 0 to 11 only.

At step s3, the UT 1 transmits the constrained reference CREF to the SBS1. Instead of using the reference CREF provided by the user terminal,the reference CREF can alternatively be generated at SBS 1, and isreferred to herein as CREF′. For example, referring to FIG. 15, at steps4, the SBS 1 generates a reference CREF′ which is, for example, anumber between 0 and n−1. At step s5, CREF′ is transmitted by SBS 1 toUT 1 to inform UT 1 of the multiplexing scheme, so that it cansubsequently correctly process SACCH data received from the SBS 1.

At step s6, CREF/CREF′ is used by SBS 1 to determine the transmissiontime of SACCH and SID blocks for each user terminal. As describedpreviously, data bursts for diverse paths can be sent using time slotpairs. For example, FIG. 16 to shows the first 12 frames of theresulting DTX signal structure where CREF/CREF′ indicates that the SACCHdata for U‘I’ 1, for example allocated to channel 2, is to be sent inframe 6, using diverse paths 1 a, 1 b, where data is to be transmittedin time slot TN=0 for diversity path 1 a and time slot TN=3 fordiversity path 1 b. FIG. 16 also shows that the SACCH blocks forchannels 0 to 5 are spread randomly throughout the multiframe. The SIDbursts corresponding to the SACCH blocks are, for example, located inthe same frame in the next available burst period.

Since a new random reference is provided for each UT and therefore eachlogical channel, the SACCH data is spread uniformly over the availableslots thus decreasing the power ripple at the satellite when DTX isoperational.

The reference used to determine burst transmission timing does not haveto be truly random but could be pseudo-random or could be apredetermined sequence of numbers which serves to distribute the SACCHbursts over the available frames. It could also be chosen in dependenceon existing power ripple at the satellite power bus.

While the SACCH multiplexing in accordance with the invention has beendescribed specifically in relation to the DTX mode, this multiplexingscheme could also be used in non-DTX mode as an alternative method ofproviding a TCH/SACCH structure. In this case, SID frames are nottransmitted and the TCH data is sent in slots not occupied by SACCHdata.

1. A method of data multiplexing in a mobile telecommunications systemin which a ground station is in communication with each of a pluralityof user terminals via a respective logical channel, each logical channelcapable of carrying link control data to a respective user terminal overdiverse communication paths using a multiple access scheme whichincludes time division, in which a frame comprises a sequence of timeslots, each time slot being associated with a logical channel, the linkcontrol data being sent in bursts in selected time slots, the methodcomprising: for each channel, setting the data burst transmission timefrom the ground station such that data bursts carrying link control datarelating to the same channel and travelling over diverse paths to thesame user terminal, arrive at the user terminal within a predeterminednumber of frames, and distributing the ground station transmission timeof data bursts carrying link control data for different channels over aplurality of frames, such that each frame which carries link controldata includes link control data relating to a single logical channelonly. 2.-22. (canceled)